1. Field of the Invention
The present invention relates to integrated circuits. More particularly, the present invention relates to an active bias circuit for low-noise amplifiers.
2. Background Information
Typically, in low-noise amplifiers (LNAs) and the like, it is desirable to reduce the contribution of noise from any component as much as possible. In bipolar circuitry, for example, a bias current is applied to the base of the transistor that comprises the LNA. However, due to process variations, temperature, and the like, the β of the transistor can vary by a factor of two, three, four or more. As a result of such varying transistor characteristics, the level of the bias current required to be applied to the transistor will change, resulting in a varying control of the gain of the LNA. Thus, under different conditions and varying transistor characteristics, the gain of the LNA can vary, which in most applications is unacceptable.
To address this problem, passive circuitry can be used to control the bias current applied to the transistor. For example, a diode device can be connected to the base electrode of the transistor. Such a diode device has an impedance proportional to 1/Gm. For a 50Ω incoming signal, such an impedance can be too low, which can result in significant signal attenuation. In addition, any noise generated by the diode device and current sources connected to it will be transferred to and affect the rest of the LNA circuit.
Consequently, a biasing scheme is needed that is independent of process variations, temperature, variations in the β of the transistor, and other like transistor characteristics.